The structural, transport, and magnetic properties of the naturally layered
manganese oxide, Sr1.6Sm1.4Mn2O7 (Sm-327), which consists of two [MnO2] bi
-layers intertwined with a rock-salt type layer of [(Sr,Sm)(2)O-2] have bee
n investigated. Both a sharp metal-insulator (M-I) transition and an antife
rromagnetic transition were found to occur simultaneously at about 118 K. E
ven in the absence of ferromagnetic ordering the resistance is suppressed r
emarkably upon applying a magnetic field. These results suggest that the M-
I transition and colossal magnetoresistance behavior observed in Sm-327 are
not driven mainly by the double exchange mechanism but ascribed rather to
the canted antiferromagnetic spin configurations of [MnO2] bi-layers, which
eventually leads to the field-induced ferromagnetic ordering. (C) 1999 Els
evier Science Ltd. All rights reserved.